The present invention relates to optics for imaging applications. It has become commonplace to employ digital imaging technology in a host of consumer electronic devices, residential systems, commercial systems and industrial applications. Often times, it is desirable to impart accurate color rendering in related devices and systems.
For color imaging arrays, it is common to cover each pixel of the array with a color spectral filter so that each pixel will respond to the color determined by its associated spectral filter. In this way, each pixel responds only to a particular color or range of colors. A system of interpolation, extrapolation and/or averaging must be used to “fill in” the two missing colors at each pixel site. For relatively large blocks of colors, this presents very little problems. However, when small detail such as created by a fine pattern in a projected image or by small spots such as created by distant headlamps or tail lamps in the projected image, the system often has serious errors in the determination of colors for small details within the image. The effects become particularly pronounced and noticeable when the pattern in the projected image is repeated with spatial frequency components which approach or exceed one half of the spatial sampling frequency for related color components in the imaging array. Larger scale interference patterns often create particularly annoying artifacts in the resulting images.
A number of approaches have been used to minimize these problems. A recent approach provides an imager for which each pixel site responds separately to each of the three color components making it unnecessary to fill in missing colors (Foveon). Such systems are proprietary and have limitations in performance. A second technique is to separate the image into separate images for each of the color components and to use individual sensors for each of these single color component images. This approach is used for high end video cameras but requires multiple imagers and a quite complicated optical system. A third approach is to intentionally blur or diffuse the image enough that the projected spots extend over several neighboring pixel areas so that the image of a small detail will never be contained in a pixel of a single color component. The problem with this approach is that unless the blur spot is very large, the percentage of the light which falls on pixels of each color will vary considerably as the image assumes various positions on the sensing pixel array. The result is that the color rendition will vary accordingly. When a very accurate color rendition is required for small details, the only apparent solution is to increase the diffusion or blurring of the image to improve the averaging. As the diameter of the blur spot increases to several pixels, the effective resolution of the sensor decreases rapidly. High resolution is normally a highly valued feature of an imager and the physical design problems and the cost of the imager and its associated image processing and image storage components increase with the number of pixels. It is important to achieve the necessary accuracy in color rendition without an unnecessary increase in the number of pixels needed to achieve a given resolution.
Very high end digital cameras often incorporate filters which include multiple layers of birefringence material to project a multiple image onto the sensor surface. To this end, the intent and the results are similar to those intended in this invention. However, the technique is very expensive and in many implementations requires a relatively thick filter assembly which often adds distortion for lenses which are not typically designed to focus the image through a thick piece of transparent material.